This invention relates to the etching of high-cubicity aluminum electrolytic capacitor foil in which the foil is first subjected to chemical etching in an aqueous acidic solution containing an acid having anions capable of anodizing aluminum, hydrochloric acid, transition metal ions, and aluminum ions. Preferably, the anodizing ions are present in a greater concentration than the chloride ions. The foil is then subjected to a second etching step which may be electrochemical or chemical. A high capacitance foil with high bend strength is produced.
Chemial etching of aluminum capacitor foil is well-known. Generally, a hydrochloric or nitric acid solution has been used with or without transition metals as catalysts. These heavy metals may be present as chlorides, nitrates, or sulfates, and may accelerate the reaction, e.g., copper salts, or retard it, e.g., iron salts.
Multi-stage etching of capacitor foil is also well-known. Frequently, the process has been electrochemical using direct and/or alternating current. When direct current is used, the electrolyte is usually a sodium chloride solution which may be acidified and may contain additives. When alternating current is used, the electrolyte is generally hydrochloric acid containing small amounts of additives such as phosphates, sulfates, tartrates, oxalates, etc.
The etch structure produced is different with each process. Chemical etching produces long straight tunnels along the [100] crystallographic direction within the aluminum foil, and these tunnels keep going in the same direction. Direct-current etching produces a branched tunnel structure reminiscent of a jungle gym with many right-angle bends in the tunnels, particularly for foils to be used at higher voltages (above 100 V). Alternate-current etching gives a completely different etch structure. It is more pitted, and the structure is reminiscent of grape clusters or cauliflower flowerets.
As the purity of aluminum increases, it is much more difficult to etch. Chemical etching of high purity foil became more difficult and, in general, it was suitable for foil intended for low-voltage use (100 V or less) which did not require etching to any great depth. Direct current etching is an easier process to control than a chemical etching process for any voltage range, and it produces a high surface area product for high-voltage use even with high-purity foil which had not been obtainable with a chemical etch heretofore. Initially, alternate current etching was used mainly for low-voltage foil. It gives a greater density of etch sites than direct current etching and frequently has been combined with it to provide a high surface area foil.
High-cubicity aluminum foil is foil in which the majority of the aluminum grains are in the [100] direction perpendicular to the foil surface. This orientation provides the possibility of producing a fairly homogenous chemical etch structure with a high density of parallel tunnels perpendicular to the foil surface. High-cubicity foil has been etched successfully using direct current, but the resulting etch structure shows etch tunnels many of which have numerous right-angle bends in the tunnels and some of which penetrate completely through the foil.